Variable pneumatic volume for cryogenic coolers

ABSTRACT

Free displacer cryogenic cooler having a free displacer regenerator positioned in a housing having a cooling end. The displacer having an opposite end in communication with a pneumatic volume; the pneumatic volume being adjustable to optimize this volume and achieve maximum cooling capability.

United States Patent 1191 Raimondi Sept. 23, 1975 [54] VARIABLE PNEUMATIC VOLUME FOR 3,472,037 10/1969 Kohler 62/6 CRYOGENIC COOLERS 3,487,635 1/1970 Prast 62/6 3,645,649 2/1972 Beale 62/6 Inventor: Peter K- Raimondi, woodbridge, 3,817,044 6/1974 Daniels 62/6 1 [73] Assignee: The United States of America as Primary ExaminerWilliam J. Wye

- represented by the Secretary of the Attorney, Agent, or FirmMilt0n W. Lee; Nathan Army, Washington, DC. Edelberg; Robert P. Gibson 221 Filed: Aug. 26, 1974 [21] Appl. No.: 500,837 [57] ABSTRACT Free displacer cryogenic cooler having a free displacer [52] US. Cl 62/6; 62/86 r rator positioned in a housing having a cooling [5 1] Int. Cl- FZSB end The displacer having an opposite end in ommu- [58] Field of Search 62/6, 86 nicati n with a pneumatic volume; the pneumatic volume being adjustable to optimize this volume and References Cited achieve maximum cooling capability.

UNITED STATES PATENTS 3,237,421 3/1966 Gifford 62/6 5 Clams 4 Drawmg F'gures US Patent Sept. 23,1975 3,906,739

FIG. 1 24 Q 22 21 ii E l8 H/ k FIG. 1b

25M 11 A/LQ FIG. 2

VARIABLE PNEUMATIC VOLUME FOR CRYOGENIC COOLERS The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND Presently, infrared detectors operate in. three major temperature ranges; 195K, 170K to 145K and "77K. The detectors in the 195K and 170K to 145K ranges are cooled by thermoelectric coolers while those at 77K must be cooled by liquid nitrogen. Nitrogen under high pressurethrough a Joule-Thompson device can'be used to cool these detectors, but this involves the everpresent danger of highpressure bottles. Vulleumier coolers or Split-Sterling devices have so far, proved infeasible due to high input power requirements and physical size. I

The use of a free displacer cooler has proven effective. In such a free displacer cooler, the displacer is the entire moving assembly while the regenerator is a heat exchanger located within the body of the displacer. The cooler functions in the following manner. As a pressure wave (generated by a compressor and applied to the cooler) reaches a peak the displacer moves upward aided by the pressure in the pneumatic volume. Convcrsely, as the pressure wave reaches a minimum the displacer returns downward but is hindered by the pressure in the pneumatic space. Therefore this pneumatic volume acts as a controlling force influencing the motion of the displacer.

It has been found though, that the size of the pneumatic volume strongly influences the temperature that can be achieved with such a cooler.

Present practice is to have this pneumatic volume machined separately and to bolt it to the cooler. Any changes made to the volume must be accomplished by making a new part. Another approach is to make a series of small pneumatic tanks and interchange these by trial and error to achieve the optimum volume needed. Attempts to determine the exact pneumatic volume theoretically, as by computer analysis, have not been satisfactory and other solutions to the problem have been sought.

SUMMARY The solution to the above stated problem, which is the basis of the disclosed invention, involves making the pneumatic volume adjustable. By varying the volume, the precise value that yields the lowest temperature at the cold head of the cooler can be determined.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la shows the invention employing a micrometer measuring device for accurately varying and measuring the pneumatic volume; v

FIG. lb shows the cooler of FIG. la with a bolt-on end cap in place of the micrometer'device;

FIG. 2 discloses a pneumatic volume control employing a screw-like device for varying the pneumatic volume;

FIG. 3 presents the cooler employing a deformable penumatic volume for volume control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS By referring to FIG. 1a, the technique for implementing the disclosed invention can be readily understood. The cooler 10 consists of a displacer'housing 11 in which the displacer 12 is positioned for reciprocating motion. Seals 13 and 14 isolate the void volume 15 from the coldend volume 16 having a cold head 17 and from the pneumatic volume 18. A further housing 19 forming the variable volume of the pneumatic volume 18 is joined in a gas-tight relation to the displacer housing 11. A piston-like member 20 is located therein and affixed to a shaft 21. This shaft is the arm of a micrometer drive 22. Sealing rings 14 and 23 seal the pneumatic volume. A line 24 carries the driving pressure pulses from a compressor (not shown) to the cooler 10.

In operation, the compressor is actuated thereby sending pressure pulses to the cooler and causing the displacer 12 to reciprocate. The position of the pistonlike member 20 is then varied by means of the micrometer drive 22 while the temperature changes resulting from the changes in the pneumatic volume 18 are observed. The micrometer reading at the lowest temperature achieved is then used to calculate the optimum pneumatic volume (the cylinder area of the variable volume being a known quantity). Of course, the micrometer drive itself could be calibrated in terms of a volume scale. Having determined the optimum volume, it only remains to provide an end member cap 25, as shown in FIG. lb, in place of the housing 19; where the end member is of the desired volume.

In FIG. 2 the cooler device 10 is constructed with a pneumatic volume 26 already a part of the housing. A screw-like element 27 is provided which projects into the volume 26. The volume 26 is purposely made slightly larger than needed. The screw element 27, when adjusted inwardly, yields the optimum pneumatic volume and is then fixed in that position as by a sealing material.

In FIG. 3, a pneumatic volume tank 30 is positioned remotely from the displacer housing 11 and is made deformable. The tank volume is varied by applying a force to deform the tank. A precision vice 31, having placer housing having a cold head end for contacting v I an object to be cooled and in which a free displacer is housing means joined in a sealed manner to said displacer housing forrning a pneumatic volume proximate the opposite end of said displacer, wherein the improvement comprises: means associated with said pneumatic volume for variably adjusting and maintaining the size of said pneumatic volume so that as the free displacer is rcciprocated by said pressure pulses, the cooling at the cold head end of the cooler can be maximized by simultaneously adjusting the pneumatic volume.

2. The device according to claim 1 wherein the means for variably adjusting the pneumatic volume comprise:

piston means in said further housing;

sealing means between the piston means and said further housing;

and drive means extending from said piston to without said further housing, whereby the positioning of said piston by said drive means varies the pneumatic volume.

3. The device according to claim 2 wherein said drive means is a micrometer shaft and wherein calibration on said shaft provide a measure of the variation of the pneumatic volume as the micrometer shaft is turned to adjust the position of said piston means.

4. The device according to claim 1 wherein the means for variably adjusting the size of the pneumatic volume comprises:

a threaded element positioned in a wall of said further housing; and

screw means inserted in said threaded element and penetrating into said pneumatic volume, whereby adjusting the penetration of said screw means into said pneumatic volume varies the volume thereof.

5. The device according to claim 1 wherein the pneumatic volume comprises a deformable structure and wherein the means for adjusting the size of said pneumatic volume comprise vise means clamping said structure, said vise means having jaws and means for varying the spacing therebetween, whereby the volume of said pneumatic volume is varied by adjusting said jaws. 

1. A FREE DISPLACER CRYOGENIC COOLER INCLUDING A DISPLACER HOUSING HAVING A COLD HEAD END FOR CONTACTING AN OBJECT TO BE COOLED AND IN WHICH A FREE DISPLACER IS POSITIONED TO RECIPROCATE IN RESPONSE TO PRESSURE PULSES PROVIDED BY A REMOTE COMPRESSER CONNECTED BY CONDUIT MEANS TO A REGION OF SAID DISPLACER HOUSING SEALED FROM SAID COLD HEAD END BY SEALING MEANS AND FROM THE END OF SAID DISPLACER OPPOSITE THE END PROXIMATE SAID COLD HEAD END BY FURTHER SEALING MEANS, AND FURTHER HOUSING MEANS JOINED IN A SEALED MANNER TO SAID DISPLACER HOUSING FORMING A PNEUMATIC VOLUME PROXIMATE THE OPPOSITE END OF SAID DISPLACER, WHEREIN THE IMPROVEMENT COMPRISES: MEANS ASSOCIATED WITH SAID PNEUMATIC VOLUME FR VARIABLY ADJUSTING AND MAINTAINING THE SIZE OF SAID PNEUMATIC VOLUME SO THAT AS THE FREE DISPLACER IS RECIPROCATWED BY SAID PRESSURE PULSES, THE COOLING AT THE COLD HEAD END OF THE COOLER CAN BE MAXIMIZED BY SIMULTANEOUSLY ADJUSTING THE PNEUMATIC VOLUME.
 2. The device according to claim 1 wherein the means for variably adjusting the pneumatic volume comprise: piston means in said further housing; sealing means between the piston means and said further housing; and drive means extending from said piston to without said further housing, whereby the positioning of said piston by said drive means varies the pneumatic volume.
 3. The device according to claim 2 wherein said drive means is a micrometer shaft and wherein calibration on said shaft provide a measure of the variAtion of the pneumatic volume as the micrometer shaft is turned to adjust the position of said piston means.
 4. The device according to claim 1 wherein the means for variably adjusting the size of the pneumatic volume comprises: a threaded element positioned in a wall of said further housing; and screw means inserted in said threaded element and penetrating into said pneumatic volume, whereby adjusting the penetration of said screw means into said pneumatic volume varies the volume thereof.
 5. The device according to claim 1 wherein the pneumatic volume comprises a deformable structure and wherein the means for adjusting the size of said pneumatic volume comprise vise means clamping said structure, said vise means having jaws and means for varying the spacing therebetween, whereby the volume of said pneumatic volume is varied by adjusting said jaws. 